This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 11-053041, filed Mar. 1, 1999, the entire contents of which are incorporated herein by reference.
The present invention relates to a nuclear medicine diagnostic apparatus for repeatedly measuring, while varying the angles of detectors relative to a subject, gamma rays which are emitted from RI (Radio-Isotope) injected into a subject and reconstructing an RI distribution (hereinafter referred to as a sectional image) based on data collected thereby. This type of nuclear medicine diagnostic apparatus is called as an ECT (Emission Computed Tomography) to be distinguished from an X ray computed tomography apparatus.
In the ECT, there are a method using a single photon RI such as 99mTC and 111In and a method using a positron RI such as FDG, 11C and 13N. The former is called a SPECT (Single Photon Emission Computed Tomography) and the latter as a PET (Positron Emission Computed Tomography). The positron emitted by a beta disintegration from the positron RI used in the PET is annihilated through a combination with an electron. At this time, two annihilated photons (gamma rays) of 511 keV are radiated in mutually opposite directions. The PET apparatus selects the paired gamma rays based on the coincidence and has the function of locating the position of the presence of the positron RI and, accurately, the annihilation position of the positron.
FIG. 1 is a view diagrammatically showing a structure of a conventional PET apparatus. In order to detect paired annihilation gamma rays emitted in opposite directions, two detectors 1, 2 are arranged opposite to each other with a subject P interposed. Parallel multi-hole collimators are mounted on these two detectors 1, 2 so as to guide only gamma rays incident at a vertical or near-vertical angle to the two detectors. Not only paired annihilation gamma rays derived from the same annihilation phenomenon but also other gamma rays are randomly incident on the two detectors. Through the utilization of the coincidence between the output of one detector 1 and that of the other detector 2, a coincidence circuit selects the paired annihilation gamma rays and counts them at each incidence position. The count operation is interruptedly repeated while varying the angles of the detectors 1, 2 relative to the subject P on a little-by-little basis. Based on the data acquired through the repeated count operation, the RI density distribution (hereinafter referred to as a sectional image) of a sectional plane is generated by a reconstruction circuit 4. This sectional image is displayed on a display unit 5.
As this type of PET, some types have the function of generating a sectional image for positioning. The sectional image for positioning is shorter in counting time than the sectional image for diagnosis and coarser in matrix. In comparison with the sectional image for diagnosis, the sectional image for positioning shortens, to an about 1/n (n: natural number), a time from the data collection to an image display.
In order to achieve positioning, it is necessary to take a plurality of sectional images different in data collection positions. And it takes a long time to achieve such positioning.
An object of the present invention is to enable a planar image to be utilized for a multi-purpose application, such as positioning, in a nuclear medicine diagnostic apparatus for reconstructing an RI distribution of a sectional plane of a subject, coincidence PET apparatus etc.
The present invention calculates the incidence directions of gamma rays by an incidence direction calculating circuit on the basis of an incidence positions of the gamma rays on the detectors. A planar image generating circuit generates planar image data on the basis of count data relating to an event coincident or near-coincident with the incident direction designated by an operator on an input device. A display unit displays a planar image on the basis of the planar image data generated.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.